Direct access storage devices (DASD) have become part of every day life, and as such, expectations and demands continually increase for greater speed for manipulating and holding larger amounts of data. To meet these demands for increased performance, the mechanical assembly in a DASD device, specifically the Hard Disk Drive (HDD) has undergone many changes.
In order for an HDD to hold more data, advances in the magnetic recording heads as well as the disk media on which the data is written have undergone major advances in the past few years. A critical relationship between the head and disk is the spacing between their adjacent surfaces. This is typically known as the fly height.
The head flies above the disk by virtue of an air film created by the disk spinning next to a pattern on the surface of the slider (and magnetic recording head contained there within). This pattern on the slider is known as the Air Bearing Surface, or ABS. The ABS is fabricated on the surface of the slider that is closest to the disk. Typically the closest point on the ABS to the adjacent disk surface resides on the magnetic recording head. Typically the head resides at the end of the slider known as the trailing edge of the slider, so called the trailing edge because it is the last edge of the slider to fly over the disk.
Control of the fly height is critical to the density of data that can be written onto the disk surface. Fly height today is in the range of 5-15 nm. If heads fly too high, data might not be transferred to and from the disk with adequate amplitude, or signal strength. If heads fly too low, there exists the potential for catastrophic failure known as head crash. As the name implies, head crash is that situation in which the head makes contact with the disk. This can result in either damage to the head, or to the disk, or to both. A head crash can result in loss of data and/or rendering the HDD inoperable.
As with any manufactured assembly, there are many tolerances and dimensions that affect the fly height of the head above the disk. One solution to low flying sliders is to tighten the tolerances and distributions of those features of all components that effect fly height. However, many of the tolerances are associated with components that support the head or the disk and are very difficult and costly to control for the head manufacturer or disk manufacturer. Moreover, in many cases it is possible to have each distinct component that affects fly height in an HDD meet its individual dimension and tolerance, but when assembled with other components the resulting fly height of one or more sliders is unacceptable due to the interaction of tolerances. Conversely, many components scrapped for failing to meet the component specifications, may not fail the final HDD test if the components are properly matched during assembly. Therefore, screening components based on component specifications in most cases is very costly and may not be very effective.
A second solution for addressing fly height discrepancies is the utilization of testers that can test for potentially low fly height. The primary testers available are “Fly Height Tester” (FHT) and “Dynamic Electric Tester” (DET). FHT is typically an apparatus that is custom built to suit the needs of a particular company. The DET is a marketed apparatus that measure the magnetic performance of a head flying over a spinning disk. Both the FHT and DET can be used to derive a value, usually in nanometers (nm) of the fly height of an individual slider.
For example, FHT produces an actual fly height measurement based on interference fringes, which result from reflecting light off the ABS and the disk surface above which the slider is flying. The DET infers fly height from the magnetic performance and amplitude of a head during track following. Both testers require complex and costly support hardware such as optics systems, servo devices, electronics and/or precision tooling.
Another problem with FHT and DET testing is cost. The high cost of an FHT and DET make 100% testing of all produced HGAs (and sliders) costly and most of the time impractical. To this end, fly height testing is typically performed on a sample basis.
What is needed in the HDD industry is means to quickly and economically detect, at the slider level, sliders that are prone to low flying.